This invention relates to a marking ink composition which exhibits an excellent resistance to drying of a nib of a pen so that the nib undergoes no significant drying even when the pen is left to stand with its cap removed.
Marking ink compositions, i.e., oil-based inks have widely been used, since they not only can be used for writing on the surface of absorbent materials such as paper or cloth as well as non-absorbent materials such as plastic, glass or metal, but also are quick-drying. These inks include various types ranging from ordinary ones capable of providing firmly fixed writings to erasable inks for writing boards which, due to the incorporation of a release agent, can provide writings on non-absorbent surfaces capable of being erased by softly rubbing with a dry cloth, etc.
However, each of the above-mentioned marking ink compositions has several problems. That is, when the ink of this type is used in writing instruments and left to stand with its cap removed, the evaporation of an organic solvent in the ink composition occurs so that a colorant, a resin and the like in the composition are deposited. Such deposition causes logging of the inside as well as the surface of the nib of the pen. As a result, the ink is not sufficiently discharged from the nib to thereby give a scratchy line in writing and further to be impossible to write.
In order to solve such problems, various proposals have been made. For example, the addition of a higher fatty acid ester of a polyglycerol to a marking ink has been proposed (see Japanese Patent Laid-Open Specification No. 58-108270), and the effect of suppressing the evaporation of a solvent from a nib by virtue of such an addition is disclosed therein. However, polyglycerols of the polyglycerol fatty acid ester used in the above-mentioned prior art are limited to those of dimer to hexamer of glycerol. Moreover, the effect of suppressing the evaporation obtained by the addition is not necessarily satisfactory from the practical point of view.